Major amnesia revamp

Instead of forgetting maps and objects, make amnesia forget skills.
Forgetting maps and objects could be circumvented with taking notes,
or by using an external tool to remember the forgotten levels.

Forgetting skills allows the player to optionally go down another
skill path, if they trained the wrong weapon in the early game.

Amnesia still forgets spells.

As a replacement for the deja vu messages when entering a forgotten
level, those messages will now indicate a ghost with your own name
existing on the level, given only when the level is entered for
the first time.

These changes based on fiqhack, with some adjustments.

src/spell.c

@@ -397,8 +397,6 @@ learn(VOID_ARGS)
             book->spestudied++;
             exercise(A_WIS, TRUE); /* extra study */
         }
-        /* make book become known even when spell is already
-           known, in case amnesia made you forget the book */
         makeknown((int) booktype);
     } else { /* (spellid(i) == NO_SPELL) */
         /* for a normal book, spestudied will be zero, but for